Brad,
> I then took a closer look at two objects. The first is located roughly at
> 512,143. EXTRACTOR says it has an ellipticity of 0.029, and PSF says it
> has an ellipticity of 0.0826. If I use the Pick Object functionality of
> GAIA I get an ellipticity of 0.045. If I take two slices in X and Y
> through the object and put both of these through FIGARO/FITGAUSS I get an
> ellipticity of 0.032.
Just had a quick look with GAIA, EXTRACTOR, and PSF.
These point sources are undersampled. In good seeing, even interleaving
doesn't achieve full sampling. For such blocky images, ellipticity has
significant errors. The image centres are dominated by off-centre
brightest pixels.
While using GAIA I was able to reproduce your measurements, I found
different results with PSF and EXTRACTOR (default settings). The
latter has so many tuning options---your's is presumably tuned to UKIRT
use---I'm not at all surprised that I obtained values about 25%
different from yours.
Still I don't know why PSF generates such a flattened shape for your
second test object. That's way more than the expected noise. That
would need detailed examination of the calculations.
My experience with such data (coarse PDS scans of Schmidt plates) before
we had the technology to process 4k-square images, makes me favour the
moments approach. It gave reasonable results as I recall, although my
focus was on the galaxies more than the stars, based upon the
star-galaxy separations.
One other test I'd make. Plot the distribution of orientations of
the point sources. This ought to be even allowing for the scatter. If
you have a image-quality problem there should be a peak. How does PSF
compare with EXTRACTOR for this histogram?
Malcolm
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